Limiting amplifier



Dec. 26, 1933. 5 B, $M|TH 1,940,735

LIMITING AMPLIFIER Filed NOV.- 15, 1929 INVENTOR SIDNEY BERTRAM SMITH BY wi/mw ATTORNEY Patented Dec. 26, 1933 oNirso STATES 1,940,735 LIMITING AMPLIFIER Sidney Bertram Smith, Gordonville, England, assignor to Radio Corporation of America,a corporation of Delaware Application November 15, 1929, Serial No: 407,314, and in Great Britain December 13, 1928 6 Claims.

This invention relates to thermionic limiting,

amplifying and the like devices, and has for its principal object to provide improved amplifying apparatus whereby the, amplitude of signals or the like may be limited.

A well known method whereby the amplitude of signals may be limited is to apply them between gridand filament of a triode whose filament is operated at substantially less than full emission, so that the saturation current ofthe valve is reduced and voltage amplification diminished on all signal amplitudes above a predetermined value. This type of limiter, however, suffers from the defect that it is not practically convenient or sometimes even possible to obtain a sufficient amplification of signals whose amplitudes are less than the predetermined value, owing to the fact that the inter-electrode capacity of triodes prevents their being employed incascade in a sufficient number of amplifying stages.

While inter-electrode capacity neutralizing arrangements are well known, they are not readily and satisfactorily applicable to limiting amplifiers owing to the fact that the limited signal amplitude may be, and often is, comparable to that or" the carry through of the valve, and consequently, for efiicient operation, a very exact degree of balance accuracy is required if the unlimited component is to be reduced to a negligible value.

According to this invention a limiting amplifier comprises a plurality of so-called screened grid thermionic valves connected in cascade and operated with reduced filament emission, whereby the saturation currents of said valves are reduced.

The screened grid valve comprises, as is well known, a filament, a control grid, a screening grid and an anode, the saidscreening grid being located between anode and control grid so as electrostatioally to screen the one from the other, and being maintained at some suitable relatively fixed potential.

Preferably, the screened grid valves, operated with reduced filament emission, are not in direct cascade, but in indirect cascade, and are alternated with ordinary amplifiers. With such an arrangement, the considerable advantage is obtained that the first limiter deals with signal variations up to a given amount (10 T. U. say), a gain (that due to the first amplifier) is coupled to the output thereofithe second limitertakes charge of a further variation in signal level (another 10 T. U. say) and so on.

The novel features which I believeto be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically one circuit organization whereby my invention may be carried into effect.

In the drawing, I I

Fig. 1 diagrammatically shows a circuit embodying the invention,

Fig. 2 graphically shows the operation of the first tube of Fig. 1, v Fig. 3 graphically illustrates the operation of 7 the entire circuit of Fig. l. I

Referring to Fig. 1 which shows diagrammatically a limiting amplifier in accordance with this invention, the said amplifier comprises three screened grid valves 1, 2 and 3 connected in cascade. The signals to be amplifiedand limited are applied across an impedance 4 connected between filament 5 and control grid 6 of the first valve. The filament 5 is operated at adull heat so as to reduce the emission. Am'plified, limited signals are tapped on across an impedance"? in the anode circuit of said valve; and applied through a coupling condenser 8 between control grid 9 and filament 10 of the second valve.

The said second valve is operated as a pure magnifier with normal filament emission, and its amplified output is applied between controlgrid and filament of the third valve, which is arranged as the first valve and similarly operated with a dull filament. Ifdesired, further amplifying and limiting screened grid valve stages may be provided. The remainder of the circuit shown will, it is thought, be self evident from the drawing.

With an arrangement of this kind, if a graph connecting input and output voltages for the first valve be drawmsaid graph having as ordinates the voltages v2 occurring across the impedance in the anode circuit of the valve, and as abscissa the voltages in, applied across theimpedance in the. grid circuit thereof, the curve shown in Fig. 2 will be obtained, which, as will be seen, rises more or less steeply (depending on the valve characteristics) from the origin until a finite value corresponding to saturation is reached, the curve then proceeding as a substantially horizontal straight line. The point 1) represents the point to which the valve is set, and the curve S represents a full wave of applied potential.

If the voltage across the output impedance of this valve be applied between control grid and filament of a similarly arranged limiting and amplifying valve, the graph connecting input voltage to the first valve with output voltage from the second will differ from the curve already described (which curve is shown also at 1 in Fig. 3) only in that the steepness of the rising portion of the curve will have been increased as shown at 2 in Fig. 3. Similarly, by the addition of further limiting amplifying stages, this steepness may be still further increased as shown at 3 in Fig. 3, so that a very high degree of sensitivity to input signals may be obtained simultaneously with a satisfactory limitation of output signals, without the use of complicated and delicate neutralizing arrangements, and with negligible interelectrode carry over.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:-

v1. A limiting amplifier for code signals comprising a plurality of screen grid tubes, said tubes being aperiodically coupled in cascade, and means maintaining the cathode emission of the cathodes of at least the first and last tubes, reduced below normal throughout operation of the amplifier.

2. In combination, in an amplifier, a plurality of screen grid tubes aperiodically connected in cascade, the tube cathodes being energized to emit electrons, and means maintaining the emission of alternate tubes reduced below normal throughout operation of the amplifier.

3. A high frequency limiting amplifier comprising in combination, a pair of screen grid tubes in cascade, an impedance connected between the control grid, and cathode of the first tube and adapted to have high frequency signals to be amplified impressed thereacross, means maintaining said cathode energized at a predetermined re duced electron emission point throughout operation of the amplifier, a coupling network connected between the output of said first tube and the input of the second tube, means maintaining I the cathode of the second tube energized at a predetermined reduced electron, emission point throughout said operation, and a second impedance connected between the anode and cathode of the second tube.

4. A high frequency limiting amplifier comprising in combination, a pair of screen grid tubes in cascade, an impedance, connected between the control grid and cathode of the first tube and adapted to have high frequency signals to be amplified impressed thereacross, means maintaining said cathode energized at a predetermined reduced electron emission point throughout operation of the amplifier, a coupling network connected between the output of said first tube and the input of the second tube, means maintaining the cathode of the second tube energized at a predetermined reduced electron emission point throughout said operation, and a second impedance connected betwen the anode and cathode of the second tube, said coupling network including a screen grid tube, and means maintaining the cathode of the latter energized throughout said operation at its normal electron emission point.

5. A high frequency limiting amplifier comprising in combination, a pair of screen grid tubes in cascade, an impedance connected between the control grid and cathode of the first tube and adapted to have high frequency signals to be amplified impressed thereacross, means maintaining said cathode energized at a predetermined reduced electron emission point throughout operation of the amplifier, a coupling network 'con nected between the output of said first tube and the input of the, second tube, means maintaining the cathode of the second tube energized at a predetermined reduced electron emission point throughout said operation, and a second impedance connected between the anode and cathode of the second tube, and means maintaining. the control grids of the tubes biased at a potential different than the cathodes thereof.

6. In a high frequency signal amplifier, adapted to amplify only signals above a predetermined SIDNEY BERTRAM SMITH. 

